There are many chemical, biological, and explosive threats that can be readily prepared and delivered in letter or flat mail pieces. Systems have been developed to extract and detect threat material contained within such mail pieces. These systems typically rely upon a series of opposing belts and rollers to pinch the mail piece and expel material contained within it. While the precise mechanism for particle expulsion has not been fully characterized, practical design experience and testing have demonstrated that a significant amount of threat material can be aerosolized and released from an envelope when passed through pinch rollers or belts. It is believed that as a letter passes through a set of pinch rollers, an air pocket forms that is eventually expelled through the end of the letter. The motion of the air in the letter acts to aerosolize threat material that is subsequently expelled from the envelope with the air.
FIG. 1 shows a prior art system in which a set of opposing pinch rollers are used to liberate particles. In this system, a piece of mail comes into contact with an initial set of opposing pinch wheels. The purpose of this first set of pinch wheels is to partially evacuate envelopes which have a relatively large amount of air trapped inside of them. This is necessary to prevent them from popping when traveling through the second set of pinch wheels. The gap between adjacent wheels allows a portion of the entrapped air within the flat envelope to shift into the pockets between adjacent wheels and gradually seep out of the envelope.
Once the envelopes have passed through the first set of pinch wheels they are immediately captured by the second set of pinch wheels. These wheels have a much greater force and do not have any gaps between adjacent wheels. Any remaining air within the envelopes is squeezed out the back of the envelope and into the air cavity between the two sets of wheels. If any hazardous materials are present in the envelope, a portion of them will be aerosolized and liberated during this process and will remain temporarily suspended in the air cavity between the two sets of pinch wheels. The aerosolized hazardous materials are then evacuated from the cavity between the two sets of wheels and passed along to analysis equipment. Detailed description of such belt-and-roller particle detection systems can be found in, for example, U.S. Pat. Nos. 6,941,794, 7,110,422 and 7,114,369.
While the belt-and-roller systems have been deployed and have proven to be effective, they generally cannot dislodge material contained on the exterior of an envelope. Moreover, the physical contact pinching mechanisms may damage the contents of a mail package. Therefore, there still exists a need for detection systems with improved performance.